This is a well worn phrase I'm trying to avoid in my writing. I'm looking for another word to replace 'hum', which I take to mean the background noise resulting from people talking. I came up with 'murmur', but ideally, I'd like a word that suggests a deep pitch. You see, I'm writing about a group of men, and in my head, I compare the sound of their voices to sounds stones would make as they bounce off each other while being carried along by a river. 'Trundle'? I thought 'gravel', but then, I plan to use that word a short distance away, and I hate clumsy repeats.